The Electro-retinogram (ERG).
    The ERG is a small response that is generated by the retina in
    response to light. The response recorded provides information about
    how well the photo receptors are changing light into a signal that
    passes to the optic nerve.

    Rod and cone function can be differentiated, as well as and inner and
    outer retinal function, by changing wavelength, intensity and duration
    of a flash stimuli under different states of dark and light adaptation.
    The recorded ERG waveform is a summation of retinal potentials from
    all retinal areas, which have different polarity, latency and amplitude.

    An ERG can also be recorded in response to an alternating
    checkerboard pattern (pattern ERG). This provides information about
    the part of the retina that images are focused on (macula).
Electro retinogram
The Electrophysiology Clinic 42 Harley St London